Generate real resized thumbnails for image attachments (#98)

The attachment grid's <img> thumbnail pointed at the same
download_attachment.php URL as the full-size original, so previewing a
multi-MB photo attachment cost a full multi-MB download just to render a
small grid preview. loading="lazy" only deferred off-screen images; it
never reduced per-image transfer size.

Generate a resized JPEG thumbnail (longest side capped at 300px) via GD
at upload time, from the same metadata-stripped image stripImageMetadata()
already produces, reusing its decompression-bomb guard (~40MP decode cap).
Store the thumbnail's filename in a new nullable ticket_attachments.
thumbnail_filename column (migration 005); NULL means no thumbnail exists
(non-image, GD unavailable, or an attachment predating this change) and
callers fall back to the full-size original.

download_attachment.php serves the thumbnail when requested via
?thumb=1 and one exists, falling back to the original otherwise. The
attachments grid now requests thumb=1 for its <img> preview; the
lightbox link is unchanged and still opens the full-size original.
delete_attachment.php removes the thumbnail file alongside the original,
and cleanup_orphan_uploads.php's orphan lookup now also matches
thumbnail_filename so generated thumbnails aren't swept up as orphans.

Verified against real MariaDB + GD: a 1600x1200 test JPEG produced a
300x225 thumbnail at ~1.8KB vs. the 52KB original (~29x smaller);
confirmed the serving logic picks the thumbnail for image attachments
with one, falls back to the original for a non-image attachment even
when thumb=1 is requested, and that the updated orphan-cleanup lookup
matches both the original and thumbnail filename (and correctly finds
neither for an unrelated filename).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117oBw2jN4kALYeS8HPq4zV

api/delete_attachment.php

@@ -94,6 +94,15 @@ try {
         }
     }
 
+    // Delete the generated preview thumbnail alongside the original, if one exists.
+    if (!empty($attachment['thumbnail_filename'])) {
+        $thumbPath = $uploadDir . '/' . $attachment['ticket_id'] . '/' . $attachment['thumbnail_filename'];
+        $realThumbPath = realpath($thumbPath);
+        if ($realThumbPath !== false && strncmp($realThumbPath, $uploadDir . DIRECTORY_SEPARATOR, strlen($uploadDir) + 1) === 0) {
+            @unlink($realThumbPath);
+        }
+    }
+
     // Delete from database
     if (!$attachmentModel->deleteAttachment($attachmentId)) {
         ResponseHelper::serverError('Failed to delete attachment record');